Process of preparing toluic acids



July 5, 1955 C. M. HIMEL ET AL PROCESS OF PREPARING 'IOLUIC ACIDS FiledMay 18, 1954 REACTION VESSEL XYLENE II 0 l9 9 1 1 J COOLER AIR ! TERPHTHALIC ACIDS MO AIR FILTER TOLUIC ACIDS.

INVENTOR CHESTER M. HIMEL, LESTER P. BERRIMAN.

ATTO

United States Patent PROCESS or PREPARING TOLUIC ACIDS Chester M. Himel,Menlo Park, and Lester P. Berriman,

Palo Alto, Calif., assignors to Richfield Oil Corporation, Los Angeles,Calif., a corporation of Delaware Application May 18, 1954, Serial No.430,748

Claims. (Cl. 260-524) This invention relates to improvements in themanufacture of toluic acids by the liquid phase oxidation of thecorresponding xylenes in the presence of a catalyst through utilizationof an oxygen-rich gas as the oxidizing medium. More particularly, ourinvention is concerned with a method of separating the toluic acidsproduced from this reaction procedure and returning the catalyst to theoxidation reaction dissolved in unreacted xylenes.

It is known to prepare toluic acids by oxidizing xylenes in the liquidphase using as a catalyst a cobalt, manganese or cerium salt which issoluble in the reaction mixture. The oxidation is carried out underelevated conditions of temperature and pressure in the presence of airor other oxygen-rich gas used as the oxidizing medium. The individualxylene isomers or mixtures thereof can be used as a starting material.

It has been proposed to separate toluic acids from the reaction mixtureof the air oxidation of xylenes by transforming the catalyst to aninsoluble state and then filtering it from the reaction mixture. Thereaction product is distilled to separate the desired toluic acids fromunreacted xylenes. Under some conditions this operation is not desirablesince it involves the relatively costly distillation step. Our presentinvention provides a means for separating the toluic acids produced bythis reaction not necessitating the performance of this distillationoperation.

We have now found that toluic acids can be separated from the reactionproduct from the liquid phase air oxidation of the xylenes by taking astream of the reaction mixture from its vessel and passing it in contactwith a free oxygen-containing gas to a cooling zone. In this zone theproduct is rapidly cooled to a temperature below 100 C. The product isthen filtered at a temperature of about 40 to 75 C. to separateprecipitated toluic acids. The filtrate comprising unreacted xylenes andcatalyst is then recycled to the oxidation reaction. Thus through thisprocedure the toluic acids produced can be separated from the catalystand unreacted xylenes in the filtrate without converting the catalyst toan insoluble form and without distilling the toluic acids as an overheadproduct to separate them from unreacted xylenes. Also the catalyst andxylenes are recovered in convenient form to be recycled to the principalreaction zone.

The oxidation of the xylenes is effected in the liquid phase at elevatedtemperatures and pressures in the presence of asoluble cobalt, manganeseor cerium salt catalyst and air or other oxygen-containing gas.Generally, the reaction temperature will range from about 130 to 250 C.,preferably 150 to 190 C., while the operating pressure will vary fromabout 50 to 500 p. s. i. g., preferably 100 to 250 p. s. i. g., and thepressure is sufficient to maintain the reaction in the liquid phase.Details concerning the oxidation of xylenes to toluic acids are setforth in Chester M. Hitnels application 2 ,712,551 Patented July 5, 1955ice Serial No. 296,718, filed July 1, 1952, now Pat. No. 2,696,499. Inpassing a stream of the reaction mixture from the reaction vessel to thecooling zone the catalyst must not be precipitated. Thus we maintain thestream in contact with an oxygen-containing gas while it is beingcarried to the cooling zone. In this zone the reaction stream must becooled rapidly to below about C. to avoid converting appreciablequantities of the catalyst to insoluble form since it is not feasible tokeep the stream in contact with sufficient oxygen-containing gas duringslow cooling to avoid conversion and precipitation of the catalyst. Therapid cooling can most advantageously be effected by flashing thereaction mixture at a pressure not exceeding about atmospheric withoutprecipitating the catalyst.

The reaction mixture from the cooling zone is then filtered at atemperature of about 40 to 75 C., and the toluic acids are separated asa solid product. If any phthalic acids are produced during the oxidationreaction, they will be precipitated along with the toluic acids, but thelatter do not decrease the value of the toluic acids in situations wherethe toluic acids are to be oxidized to phthalic acids. The filtrateincludes unreacted xylenes having dissolved therein the oxidationcatalyst. The catalyst is returned to the reaction zone in the recycledfiltrate.

If it be desirable to obtain the toluic acids uncontaminated withphthalic acids, our process can be varied to produce this result. inthis variation a filter is placed in the line conveying the reactionmixture from the oxidation vessel to the cooling zone. By filtering thestream at a temperature of about to 160 C. in the presence of anoxygen-containing gas, the catalyst and toluic acids present remaindissolved and the phthalic acids are removed as a solid. Since thephthalic acids are separated from the reaction mixture, they will notseparate as solids along with toluic acids in the filtration step afterthe rapid cooling step, and thus the toluic acids produced areuncontaminated with phthalic acids. Where oxidation temperatures above160. C. are employed, the reaction mixture passing to the filter whichremoves the phthalic acids must be cooled as in a heat exchanger so thatat the time of filtrationthe temperature of the mixture will not beabove about- 160 C.

This invention will be described in further detail with reference to theaccompanying drawing showing by a flow sheet a method of efiecting ourprocess.

Example I In reaction vessel 2 there is maintained unreacted paraxylene,a pressure of 200 p. s. i. g. and a liquid level of reaction mixturehaving a temperature of to C. The reacting mixture contains about 0.5weight per cent of cobalt toluate. Air at the rate of 500 cubic feet perhour (measure at 60 F. and one atmosphere) is charged through line 1.OiT-gases from the reaction are removed by means of line 3 intocondenser 4. Condensed paraxylene is separated from the waterand removedthrough line 6 to the xylene charge tank 7. Off-gases are vented throughline 5. Water is taken from the bottom of condenser 4. Oxidationproducts at a rate of 620 lbs/hr. are pumped through line 9 in which airis injected through line 10 to maintain a concentration of oxygen duringthe filtration which occurs in continuous filter 11 which removesphthalic acids. The efiluent liquid from the filter is then flashed incooler tank 12 maintained at 2 p. s. i. a. Vapors from the cooler areconveyed by line 13 and condensed in condenser 14, and separatedparaxylene is returned by way of line 17 to tank 7. Water is removedfrom condenser 14 by line 15 while off-gases pass through line 16. Thecrystallized toluic acids in tank 12 at 70 C. are filtered continuouslyin filter 18 and washed with pounds per hour of make-up paraxylene fromtank 7 by way of line 19. The total liquid efiluent stream containingcatalyst at a temperature of 38 C. from filter 18 is pumped back intothe reactor 2 by way of line 20 to maintain the temperature of thereaction mixture at 150 to 155 C. Filtered and washed paratoluic acidsin an amount of about pounds per hour are discharged from filter 18.Paraxylene is supplied to make-up tank 7 by way of line 8.

Example 11 The process of Example I is carried out except that filter 11is removed from line 9. In this case the phthalic acid is isolated withthe toluic acid from filter 18. This product is suitable for subsequentconversion to phthalic acid by HNO3 oxidation.

Example III Commercial meta-paraxylene is crystallized in a conventionalbatch apparatus to give the first crystalline fraction which isrecovered by filtration. This fraction has the following analysis:paraxylene 70%, meta-xylene 25%, orthoxylene 3%, ethylbenzene 2%, and isused without further purification. This feedstock is charged to theprocess of Example I and the crystalline toluic acid product recoveredcontaining orthotoluic acid and benzoic acid. The product can be used assuch or charged to a topping still to remove a mixture of benzoic acid,orthotoluic acid and a minor amount of metatoluic acid, overhead. Theresidue product is 99.5% meta-paratoluic acids (76% para-, 23.5% meta-).

We claim:

1. In the preparation of toluic acids by oxidizing the correspondingxylenes, the steps of introducing xylenes and an oxygen-containing gasinto a reaction vessel wherein xylenes are oxidized in the liquid phasemaintained under elevated conditions of temperature and pressure andcontaining as a catalyst a salt which is soluble in the reaction mixtureand which is selected from the group consisting of cobalt salts,manganese salts and cerium salts, withdrawing from the reaction vessel astream of the reaction mixture and passing it to a cooling zone while incontact with a free oxygencontaining gas to maintain the catalyst in asoluble form, cooling rapidly the temperature of the stream to belowabout 100 C., filtering the cooled materials at a temperature of aboutto C. to separate the toluic acids as solids, and recycling theoxidation catalyst in the filtrate to the reaction zone.

2. The method of claim 1 in which the xylene feed is paraxylene.

3. The method of claim 1 in which the catalyst is a cobalt salt.

4. The method of claim 1 in which the rapid cooling of the stream tobelow about C. is efiected by flashing the stream at a pressure notexceeding about atmospheric.

5. In the preparation of toluic acids by oxidizing correspondingxylenes, the steps of introducing xylenes and an oxygen-containing gasinto a reaction vessel wherein xylenes are oxidized in the liquid phasemaintained under elevated conditions of temperature and pressure andcontaining as a catalyst a salt which is soluble in the reaction mixtureand which is selected from the group consisting of cobalt salts,manganese salts and cerium salts, and in which the severity of theoxidation conditions is such that phthalic acid is formed in addition totoluic acid, withdrawing from the reaction vessel a stream of thereaction mixture, filtering the stream in the presence of anoxygen-containing gas and at a temperature from about to C. whereby thecatalyst and toluic acid remain soluble in the stream and precipitatedphthalic acid is removed, passing the filtrate to a cooling zone whilein contact with a free oxygen-containing gas to maintain the catalyst ina soluble form, cooling rapidly the temperature of the filtrate to belowabout 100 C., filtering the cooled materials at a temperature of about40 to 70 C. to separate the toluic acid as a solid, and recycling theoxidation catalyst in the filtrate to the reaction zone.

No references cited.

1. IN THE PREPARATION OF TOLUIC ACIDS BY OXIDIZING THE CORRESPONDINGXYLENES, THE STEPS OF INTRODUCING XYLENES AND AN OXYGEN-CONTAINING GASINTO A REACTION VESSEL WHEREIN XYLENES ARE OXIDIZED IN THE LIQUID PHASEMAINTAINED UNDER ELEVATED CONDITIONS OF TEMPERATURE AND PRESSURE ANDCONTAINING AS A CATALYST A SALT WHICH IS SOLUBLE IN THE REACTION MIXTUREAND WHICH IS SELECTED FROM THE GROUP CONSISTING OF COBALT SALTS,MANGANESE SALTS AND CERIUM SALTS, WITHDRAWING FROM THE REACTION VESSEL ASTREAM OF THE REACTION MIXTURE AND PASSING IT TO A COOLING ZONE WHILE INCONTACT WITH A FREE OXYGENCONTAINING GAS TO MAINTAIN THE CATALYST IN ASOLUBLE FORM, COOLING RAPIDLY THE TEMPERATURE OF THE STREAM TO BELOWABOUT 100* C., FILTERING THE COOLED MATERIALS AT A TEMPERATURE OF ABOUT40* TO 70* C. TO SEPARATE THE TOLUIC ACIDS AS SOLIDS, AND RECYCLING THEOXIDATION CATALYST IN THE FILTRATE TO THE REACTION ZONE.